(1) Field of the Invention
The present invention relates generally to adaptive signal processing, and more particularly to an adaptive signal processing system for forming an optimum signal-free reference for ultimate use in an adaptive hull-radiated noise cancellation system.
(2) Description of the Prior Art
Adaptive signal processing methods have been developed for the cancellation of submarine sonar hull-radiated noise in the near-field region of the submarine hull. The procedure typically utilizes three hydrophone pressure sensors aligned normal to the hull. The hydrophones are positioned within the near-field of any hull noise that may be generated. This is a region where the noise decays exponentially with respect to distance from the hull. It is well defined by the material properties of the hull structure and frequency range of interest as is well known in the art.
The pressure measured by the hydrophones contains both target signals and unwanted hull-radiated noise components. The adaptive signal processing procedure is based on the circuitry developed by Widrow for noise cancellation. See "Adaptive Noise Canceling: Principles and Applications," by Widrow et al., Proceedings IEEE, Volume 63, No. 12, pp. 1692-1716, December 1975. Such circuitry requires a sensor which measures both signal and noise and is referred to as the primary sensor. More importantly, a secondary input, referred to as the reference, requires a sensor that measures noise only and must therefore be "signal-free". This reference input is filtered adaptively by using the Least Mean Square (LMS) algorithm which attempts to produce an output that is a replica of the noise on the primary input. The subtraction of the filtered reference replica from the primary input then provides the cancellation of noise. A "signal-free" reference is thus an essential requirement for an effective adaptive noise cancellation system. If any portion of the signal is present on the reference channel, the signal as well as noise may be canceled adaptively. This would reduce the effectiveness of the adaptive noise cancellation system as well as any other systems that are required for post-processing of signals.
One method of generating the "signal-free" reference is disclosed by Miller in U.S. Pat. No. 4,589,137, issued May 13, 1986. Miller teaches the use of a three element line of hydrophones, or tripole, as the reference channel input source. The signal-free reference is arrived at by trial-and-error adjustment of a plurality of preamplifiers and phase shifters. Adjustment is required for each of twenty or more narrow frequency bins in order to achieve a wide band signal-free reference. This process is time consuming, is prone to operator error and typically lacks sufficient frequency resolution since the time required to generate each (frequency dependent) signal-free reference limits the number of frequencies processed.